No A/C, driving me crazy.

Hi everyone,

I could use your help. My A/C compressor won't reliably cycle, despite proper condenser airflow, proper refrigerant charge level, etc. I've ruled out the pressure switch that is attached to the drier/accumulator, and when I jump the A/C relay, it works perfectly, and low and high side pressures are within the expected range.

I gave up and took it to a local shop about two weeks ago. Their initial diagnosis was a bad A/C clutch. So, I took it home and swapped out the drier/accumulator and the A/C compressor with Genuine GM and Denso units, respectively.

I took it back to the shop to charge it with refrigerant and they called saying it still wasn't working, and they would diagnose further.

To date, they have supposedly validated that all inputs are correct, but for some reason the "on" command is not getting to the A/C compressor clutch.

Using a Tech2 type scanner they are able to command the AC compressor "on", and it works, but cannot get the system to perform this itself.

They have supposedly confirmed that all sensors are good, the HVAC control module is good, and the PCM is sending the right signal when the A/C is turned on inside the cab.

At this point they are doing continuity testing because they think it might be a broken wire, or a bad connection.

One piece of info that may be critical - it seems my A/C woes started shortly after I replaced a battery. I've done this probably close to a dozen times on this truck and it's never caused any issues before.

Any ideas of what we may need to check next?

Is this the auto control HVAC? Have you tried that recalibration procedure?

If so, have you checked that little ambient temperature probe on the driver's side headliner? It's got a little wheel and mine was seized and the temperature didn't work properly (I'm not sure if that will cause your symptoms, but worth a shot.)

Yes, attempt a reset/recal. How much refrigerant was put in?

clandr1 said:

Hi everyone,

I could use your help. My A/C compressor won't reliably cycle, despite proper condenser airflow, proper refrigerant charge level, etc. I've ruled out the pressure switch that is attached to the drier/accumulator, and when I jump the A/C relay, it works perfectly, and low and high side pressures are within the expected range.

I gave up and took it to a local shop about two weeks ago. Their initial diagnosis was a bad A/C clutch. So, I took it home and swapped out the drier/accumulator and the A/C compressor with Genuine GM and Denso units, respectively.

I took it back to the shop to charge it with refrigerant and they called saying it still wasn't working, and they would diagnose further.

To date, they have supposedly validated that all inputs are correct, but for some reason the "on" command is not getting to the A/C compressor clutch.

Using a Tech2 type scanner they are able to command the AC compressor "on", and it works, but cannot get the system to perform this itself.

They have supposedly confirmed that all sensors are good, the HVAC control module is good, and the PCM is sending the right signal when the A/C is turned on inside the cab.

At this point they are doing continuity testing because they think it might be a broken wire, or a bad connection.

One piece of info that may be critical - it seems my A/C woes started shortly after I replaced a battery. I've done this probably close to a dozen times on this truck and it's never caused any issues before.

Any ideas of what we may need to check next?

Click to expand...

Has anyone monitored the AC Request Withheld PID? This parameter lists all the prerequisites for compressor clutch operation.

strutaeng said:

Is this the auto control HVAC? Have you tried that recalibration procedure?

If so, have you checked that little ambient temperature probe on the driver's side headliner? It's got a little wheel and mine was seized and the temperature didn't work properly (I'm not sure if that will cause your symptoms, but worth a shot.)

Click to expand...

Yes, it’s auto/dual zone AC. Completed the calibration multiple times after I replaced the driver side blend door actuator.

I haven’t checked the ambient temp sensor in the headliner. It’s cheap enough I’ll give it a shot when I get the truck back.
Thank you!

OR VietVet said:

Yes, attempt a reset/recal. How much refrigerant was put in?

Click to expand...

2.5 ibs, per spec.

TJ Baker said:

Has anyone monitored the AC Request Withheld PID? This parameter lists all the prerequisites for compressor clutch operation.


View attachment 489520

Click to expand...

Thank you, I’ll ask them about this.

A reset/recal may be needed after any a/c work and not just a recent one for any other reason, like the blend door.

Key word is "may".

clandr1 said:

haven’t checked the ambient temp sensor in the headliner.

Click to expand...

Ambient temperature is outside the vehicle.

Inside the vehicle is cabin temperature.

Hauckende issue with magnetic

clandr1 said:

Hi everyone,

I could use your help. My A/C compressor won't reliably cycle, despite proper condenser airflow, proper refrigerant charge level, etc. I've ruled out the pressure switch that is attached to the drier/accumulator, and when I jump the A/C relay, it works perfectly, and low and high side pressures are within the expected range.

I gave up and took it to a local shop about two weeks ago. Their initial diagnosis was a bad A/C clutch. So, I took it home and swapped out the drier/accumulator and the A/C compressor with Genuine GM and Denso units, respectively.

I took it back to the shop to charge it with refrigerant and they called saying it still wasn't working, and they would diagnose further.

To date, they have supposedly validated that all inputs are correct, but for some reason the "on" command is not getting to the A/C compressor clutch.

Using a Tech2 type scanner they are able to command the AC compressor "on", and it works, but cannot get the system to perform this itself.

They have supposedly confirmed that all sensors are good, the HVAC control module is good, and the PCM is sending the right signal when the A/C is turned on inside the cab.

At this point they are doing continuity testing because they think it might be a broken wire, or a bad connection.

One piece of info that may be critical - it seems my A/C woes started shortly after I replaced a battery. I've done this probably close to a dozen times on this truck and it's never caused any issues before.

Any ideas of what we may need to check next?

Click to expand...

How can there be a problem with the magnetic clutch if you bypassed the compressor relay at the beginning and the system is running?

Strange diagnosis...

If the signal isn't reaching your magnetic clutch, then your system is detecting that either the pressure conditions are incorrect. High and low pressure are important here. The low-pressure switch indicates whether there is enough refrigerant in the system, and the high-pressure switch ensures that the high pressure doesn't get too high, causing the refrigerant to back up before the expansion valve or the nozzle. The high-pressure sensor would also trigger if there is too much refrigerant in the system.

In addition to the air conditioning being ON, the low-pressure switch being closed, and the high-pressure switch being closed, there are several enabling conditions:

The engine must be running.
The compressor clutch won't engage with just the ignition on.

The PCM must have an engine speed signal.

The refrigerant pressure must be within the plausible range.

Depending on the year of manufacture, the GMT 800 may have not only simple pressure switches but also an A/C Refrigerant Pressure Sensor.
... The PCM can shut off the compressor if the pressure is too high or too low.

Typical shutdowns:

Insufficient refrigerant → pressure too low
Overpressure due to overfilling or poor condenser cooling → pressure too high
Engine temperature must not be too high
In case of overheating, the PCM can shut off the air conditioning to reduce engine load.

The compressor is deactivated particularly at high coolant temperatures.

Wide Open Throttle (WOT) Detection
At full throttle, the PCM can briefly shut off the compressor.

Reason: to maximize engine performance and protect the compressor.

Blower/HVAC Request
The climate control system must actually send a cooling request.

In some GM systems, the compressor is not activated if:
only defrost is active, depending on the logic (where defrost automatically requests A/C in many GM vehicles)
the blower is off or there is no valid HVAC request.

Electrical Conditions
Fuses must be intact.

The A/C relay must be functioning.

The magnetic clutch coil must have continuity.

Sufficient battery voltage must be present.

PCM-side load limiting: The PCM can shut off the compressor in the event of:
high engine load
low battery voltage
certain fault conditions
The typical control circuit, in simplified terms, looks like this:
A/C switch → HVAC module → PCM → A/C relay → compressor magnetic clutch

Somewhere here is your problem

I would do initial testing using manual HVAC mode. That takes away any issues that AUTO can cause.

Another thing to check would be the fuses since they can get oxidation on the prongs on older vehicles that prevents the flow of electricity even though they look okay. Pulling them out and putting them back in can sometimes scrape off the oxidation or maybe just replace fuses related to the system.

Nonickatall outlined this perfectly!

I would only add that any AC shop that told you your clutch is bad on a system that engages & works normally with the relay jumped has no idea how the system works, disregard everything they told you about your system; it never should have been opened up & didn't need the compressor or dryer

My bet is a lazy AC relay, pressure switch or the control module being intermittent, jumping the pressure switches with a paper to see if the issue goes away can narrow it down some, if that doesn't work I would be looking at the control module

New pressure switches don't make assuming they're working a reasonable assumption these days, parts today are junk

Good luck!

In my experience go to the ac compressor relay and remove it. Key off, on one of the pronges u should have 12v. Constant. Then key on or crank it. Have someone cycle the ac button and check for switch on 12v. When button is pressed. If both are there then the problem is under the hood. If 12v switch is not there then low or high pressure switch or HVAC Module.

There is a high and low pressure switch. One of these is on the ac line just after it leaves the manifold of the compressor. i believe it's the high pressure switch. Check the wiring harness the contains that switches wiring startiing at switch going backwards.

Mine did the same thing. The harmess sleeve had broken down from oil exposure and the wires had been chaffing on something, I forget what it was, and there was a break in a couple of the switches wires right where they entered the connector. Liquid electrical tape was a life saver. I taped and then used heat shrink warp over each wire. No issues since.

I replaced the high pressure switch/transducer on Thursday and the A/C clutch engaged for about 3-4 minutes idling in the garage, but then it stopped again.

Yesterday I replaced the connector and ~6" of wire lead with it to the high pressure switch/transducer with GM part number PT2319, thinking along the same lines as Meathead16, that the connector and/or wiring had become compromised over 21 years of heat cycles near the coolant hoses coming off the water pump.

It worked for about 15 minutes idling in the garage and I was so happy - I thought it was fixed. I took it for a drive and it didn't even make it out of the neighborhood before the A/C clutch disengaged and it was blowing warm air.

I've given up. I'm taking it to the dealer for diag Monday morning. I contemplated spending $1500 on a Tech 2, but I'm just going to pay someone to do a proper diag rather than try to learn my way through an expensive tool I might only use a couple of times. I've been without A/C since late June and I'm over it.

To date, I've done:

1. discharge/vacuum/recharge refrigerant
2. replaced compressor, accumulator/drier, and orifice tube (no sign of gunk or metal shavings)
3. replaced low and high pressure switches (and jumped the low pressure connector to no avail)
4. replaced connector for high pressure switch
5. had it at a shop for 2 weeks who said a new compressor would fix it, but it didn't

I'll keep this thread updated - this is the first time in 21 years of ownership I've been this frustrated and at a loss on how to fix something.

edit to add:

Additionally, I should note that the PID values I can read with my inexpensive OBD II reader are all in spec, and shouldn't be telling the PCM to disengage the clutch. About the only values I can't read are cabin air temp, low pressure status, and high pressure voltage. Hoping the dealer can pinpoint one of those as a potential culprit.

I'm not affiliated with this seller but there are plenty of Tech 2s out there that are way under $500.

clandr1 said:

I replaced the high pressure switch/transducer on Thursday and the A/C clutch engaged for about 3-4 minutes idling in the garage, but then it stopped again.

Yesterday I replaced the connector and ~6" of wire lead with it to the high pressure switch/transducer with GM part number PT2319, thinking along the same lines as Meathead16, that the connector and/or wiring had become compromised over 21 years of heat cycles near the coolant hoses coming off the water pump.

It worked for about 15 minutes idling in the garage and I was so happy - I thought it was fixed. I took it for a drive and it didn't even make it out of the neighborhood before the A/C clutch disengaged and it was blowing warm air.

I've given up. I'm taking it to the dealer for diag Monday morning. I contemplated spending $1500 on a Tech 2, but I'm just going to pay someone to do a proper diag rather than try to learn my way through an expensive tool I might only use a couple of times. I've been without A/C since late June and I'm over it.

To date, I've done:

1. discharge/vacuum/recharge refrigerant
2. replaced compressor, accumulator/drier, and orifice tube (no sign of gunk or metal shavings)
3. replaced low and high pressure switches (and jumped the low pressure connector to no avail)
4. replaced connector for high pressure switch
5. had it at a shop for 2 weeks who said a new compressor would fix it, but it didn't

I'll keep this thread updated - this is the first time in 21 years of ownership I've been this frustrated and at a loss on how to fix something.

edit to add:

Additionally, I should note that the PID values I can read with my inexpensive OBD II reader are all in spec, and shouldn't be telling the PCM to disengage the clutch. About the only values I can't read are cabin air temp, low pressure status, and high pressure voltage. Hoping the dealer can pinpoint one of those as a potential culprit.

Click to expand...

You said something interesting to me. You said you "replaced the low pressure switch". Am I right assuming you are talking about the "low pressure cycling switch" that attaches at the accumulator/drier? Then you said, "jumped the low pressure connector to no avail". I am assuming you jumped with something like a jump wire with correct male ends or what I used to use.....a paperclip? If you jumped and the clutch did not engage, then there is something going on with that harness or connector or where that harness leads to. Theoretically, jumping that low pressure cycling connector bypasses the switch and forces the clutch to engage, even if no refrigerant is in the system.

OR VietVet said:

You said something interesting to me. You said you "replaced the low pressure switch". Am I right assuming you are talking about the "low pressure cycling switch" that attaches at the accumulator/drier? Then you said, "jumped the low pressure connector to no avail". I am assuming you jumped with something like a jump wire with correct male ends or what I used to use.....a paperclip? If you jumped and the clutch did not engage, then there is something going on with that harness or connector or where that harness leads to. Theoretically, jumping that low pressure cycling connector bypasses the switch and forces the clutch to engage, even if no refrigerant is in the system.

Click to expand...

Yes, but if the PCM is getting a reading it doesn’t like re: high pressure, TPS, engine coolant temp, ambient temp,Or engine idle, jumping the low pressure switch connector won’t make the clutch engage.

clandr1 said:

Yes, but if the PCM is getting a reading it doesn’t like re: high pressure, TPS, engine coolant temp, ambient temp,Or engine idle, jumping the low pressure switch connector won’t make the clutch engage.

Click to expand...

If you say so.